This invention relates to a method of manufacturing a semiconductor substrate, and more particularly a method of manufacturing a substrate having dielectric isolation regions. In recent years, a selective oxidation process has been widely used for manufacturing LSI IC devices.
FIGS. 1A to 1D show a conventional method of manufacturing dielectric isolation areas on a semiconductor substrate by using the selective oxidation process.
At first, a thermally-grown silicon oxide film 2 having a thickness of about 300 .ANG. is formed of a monocrystalline silicon substrate 1, and then a silicon nitride film 3 having a thickness of about 2000 .ANG. is formed on the silicon oxide film 2 by using a reduced pressure CVD (chemical vapor deposition) process (FIG. 1a).
After forming the photo resist pattern 4 on the surface of the silicon nitride film 3 by a conventional photolithography process, the silicon nitride film 3 and the silicon oxide film 2 are selectively etched away to form a heat resistant mask 6 (FIG. 1b).
After removing the photo resist pattern 4, the thus obtained structure is subjected to a thermal oxidation process. Since the exposed surface of the silicon substrate 1 is thermally oxidized, the thick oxide regions 7 is formed so as to surround active regions (FIG. 1c).
Then, the silicon oxide film 2 and the silicon nitride 3 are removed to expose the surface of the active element region 9 (FIG. 1d).
However, in this selective process, oxidation invades under the edges of the nitride mask 6, thereby forming a so called bird's beak 8. The bird's beak region prevents from shrinking the width of the thick oxide region 7 used as a dielectric isolation region, so that it has been difficult to increase the packing densities of IC devices. In addition, since active regions are designed in consideration of the generation of the bird's beak, it has been difficult to accurately define the areas thereof.